CdSe/TiO2 core-shell nanoparticles produced in AOT reverse micelles: applications in pollutant photodegradation using visible light
© Fontes Garcia et al; licensee Springer. 2011
Received: 31 October 2010
Accepted: 15 June 2011
Published: 15 June 2011
CdSe quantum dots with a prominent band-edge photoluminescence were obtained by a soft AOT water-in-oil (w/o) microemulsion templating method with an estimated size of 2.7 nm. The CdSe particles were covered with a TiO2 layer using an intermediate SiO2 coupling reagent by a sol-gel process. The resulting CdSe/TiO2 core/shell nanoparticles showed appreciable photocatalytic activity at λ = 405 nm which can only originate because of electron injection from the conduction band of CdSe to that of TiO2.
Over the last decade, nanostructured semiconductor materials have been the focus of intense research efforts . The striking feature of a nanometric solid is that conventionally detectable properties are no longer constant, but are tuneable by simply controlling its shape and size, and this has originated a revolution in materials science and device technology. Their photophysics shows high luminescence with tuneable emission maxima and narrow bandwidth. Semiconductor nanocrystals (CdSe, ZnS, etc.), metallic nanocrystals (Ag, Au, etc.) and magnetic nanocrystals (Ni, Fe3O4, etc.) can be prepared by templating with the aqueous cavities existent in self-organized structures of water-in-oil (w/o) microemulsions . The main aspects that control the structure of these nanoparticulate systems are the nucleation and growth processes, which are determined by the microemulsions dynamics, the interaction between nanoparticle surface, and surfactant molecules and, if needed, by the presence of metal-complexing agents. Core-shell nanoparticles (CdSe/ZnS) have also been prepared by templating techniques , opening the range of possibilities for tailoring the material to meet the specific needs of application and improving its biocompatibility. In this study, we succeeded in the production of CdSe quantum dots (QDs) with 2.7 nm size being emitted with high quantum yield at 545 nm with a halfwidth of 30 nm using AOT reverse micelles as templates and polyselenide, Se n 2-, as the selenium source. We have grown a titanium dioxide shell above the cadmium selenide core. The huge decrease observed in the photoluminescence (PL) quantum yield of the resulting particles indicates the formation of core-shell CdSe/TiO2 nanoparticles, which was reported as due to a photoinduced electron transfer from CdSe to TiO2 in a linked arrangement . This process can thus capacitate the TiO2 outer layer for electron transfer reactions with adsorbed or surrounding molecules. TiO2 can originate this photocatalytic process by itself but, due to a high band gap, UV radiation is needed with λ < 387 nm. The advantage of the prepared nanoparticles is the possibility of efficient use of visible light for the same purpose.
All the solutions were prepared using spectroscopic grade solvents. Selenium powder (99.5%) was obtained from ACROS. Cadmium nitrate tetrahydrate (98%), sodium sulphide (98%), sodium bis(2-ethylhexyl) sulfosuccinate (AOT, 99%), hydrazine, 25%(w/w) solution of tetraethylammonium hydroxide in methanol, (3-mercaptopropyl)trimethoxysilane (95%), tetra-n-butylorthotitanate were all obtained from Sigma-Aldrich. Titanium dioxide P25 was donated by Degussa. All the reagents were used as received.
Preparation of CdSe QDs
from which one can see that the relation between Se and the organic base determines the type of polyselenide that is formed. The resulting homogeneous solution has a dark green colour. For the preparation of the second microemulsion first a given amount of water is injected, then the sodium sulphide solution and finally the polyselenide/DMF solution. The resulting microemulsion solution acquired a very slight rose coloration. The total aqueous volume is similar to that of the first microemulsion. The final concentration of Cd and Se was 2 × 10-4 M. The used molar ratios were Cd/SO3 2- = 0.1, Se/hydrazine = 0.5, Se/organic base = 1.5, Se/SO3 2- = 0.1.
Preparation of CdSe/TiO2 nanoparticles
A 1:10 mixture of a (3-mercaptopropyl)trimetoxysilane (MTMS) and tetra-n-butylorthotitanate (TBOT) was directly added to the solution of CdSe QDs in AOT. This allowed for the covalent coupling of the QDs surface with silicon alkoxide through its -SH group. The water present in the microemulsion allows for a sol-gel process that results in a small initial layer of SiO2 followed by an outer shell of TiO2. The solution turned turbid and slightly gelatinous and the fluorescence previously observed for the CdSe QDs disappeared. After heating at 60°C for 45 min, a coloured precipitate settled in the bottom. The colourless supernatant was removed with a pipette, and the solid was washed several times with ethanol to remove the remaining AOT surfactant molecules. The molar ratios used were MTMS/Cd = 1, and TBOT/Cd = 10.
Absorption spectra were recorded using a Shimadzu UV-3101PC UV-Vis-NIR spectrophotometer. Fluorescence measurements were performed using a Fluorolog 3 spectrofluorimeter, equipped with double monochromators in both excitation and emission. Fluorescence spectra were corrected for the instrumental response of the system.
The irradiation setup is based on a 150-W Xe arc lamp from Lot-Oriel with appropriate interference filters (340 or 405 nm with 10 nm halfwidth) placed before the cuvette holder. A focusing lens was used so that the cuvette could be placed in focus at a distance of 42.5 cm from the lamp with a spot of 8 mm. The cuvette was filled with a 0.1 g/L dispersion of either TiO2 from Degussa or the prepared CdSe/TiO2 core/shell nanoparticles in a 1.4 × 10-5 M methylene blue (MB) aqueous solution. The light intensity at the cuvette holder was measured using a handheld power meter model 3803 obtained from New Focus. A value of 2.4 mW was obtained at 405 nm using an interference filter from Edmund Optics (20% peak transmission). From the known profile of the arc Xenon lamp and the transmission of a 340 nm interference filter, we can calculate the intensities of the lamp as 3.2 × 10-8 Einstein/cm2 s at 405 nm and 6.9 × 10-9 Einstein/cm2 s at 340 nm.
Results and discussion
Using an empirical relation , we can estimate from the first excitonic absorption peak a 2.7 nm particle size. The halfwidth of the PL is about 30 nm, which indicates that the particles are fairly monodisperse although a small red shift of the excitation spectra in relation to the absorption is observed. This comes from the fact that PLE gives the absorption of the subpopulation of particles that contribute more to the emission at the select wavelength. In order to obtain the full range of the absorption spectra, the selected emission wavelength is usually at the red-edge of the PL spectrum. This favours larger particles for which the absorption and PL occur at lower energies (quantum size effect). We thus conclude that the prepared CdSe QDs are not monodisperse but their size distribution is not large on account of the observed small halfwidth of the PL spectrum.
CdSe/TiO2 core-shell nanoparticles
Photodegradation of MB
This study was funded by the FCT-Portugal and FEDER through CFUM.
- Zhong W: Nanomaterials in fluorescence-based biosensing. Anal Bioanal Chem 2009, 394: 47. 10.1007/s00216-009-2643-xView Article
- Kortan AR, Hull R, Opila RL, Bawendi MG, Steigerwald ML, Carroll PJ, Brus LE: Nucleation and growth of cadmium selendie on zinc sulfide quantum crystallite seeds, and vice versa, in inverse micelle media. J Am Chem Soc 1990, 112: 1327. 10.1021/ja00160a005View Article
- Robel I, Kuno M, Kamat PV: Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO 2 Nanoparticles. J Am Chem Soc 2007, 129: 4136. 10.1021/ja070099aView Article
- Eggert H, Nielsen O, Henriksen L: Selenium-77 NMR. Application of JSe-Se to the analysis of dialkyl polyselenides. J Am Chem Soc 1986, 108: 1725. 10.1021/ja00268a001View Article
- Yu WW, Qu L, Guo W, Peng X: Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem Mater 2003, 15: 2854. 10.1021/cm034081kView Article
- Costi R, Saunders AE, Elmalem E, Salant A, Banin U: Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe-Au Nanodumbbells. Nano Lett 2008, 8: 637. 10.1021/nl0730514View Article
- Harris C, Kamat PV: Photocatalysis with CdSe Nanoparticles in Confined Media: Mapping Charge Transfer Events in the Subpicosecond to Second Timescales. ACS Nano 2009, 3: 682. 10.1021/nn800848yView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.